The present invention relates to a method of allocating available frequency bands to different cells in a TDMA cellular radio system wherein at least some of the receivers within the system use interference cancellation methods to cancel the effects of co-channel interference in a desired signal.
As far as narrow-band cellular radio systems are concerned, co-channel interference is one of the most significant factors limiting system capacity. The term, co-channel interference, refers to interference in the desired signal caused by signals transmitted in nearby cells on the same frequency. For this reason, cellular radio systems have conventionally been designed so that the same frequency is only used in cells located at a sufficient distance from each other, whereby the interfering signals remain within acceptable limits as a result of propagation attenuation of the signals. This leads to a re-use pattern in the cellular structure. For example, re-use pattern seven is one in which one seventh of all available frequency bands is allocated to every cell, and the same frequencies are re-used at a relative re-use distance of 21 from each other.
The system capacity is to be increased, the need arises to shorten the re-use distance without, decreasing transmission quality. Conventionally, an increase in system capacity has been achieved by decreasing cell sizes and reducing transmission power levels. Another method for solving the problems concerning capacity is to keep up with the fast development of digital signal processing, and utilize the interference cancellation algorithms developed. Cancelling cancellation of co-channel interference in the receiver enables a more efficient frequency re-use within the network.
In the present-day cellular radio systems, co-channel signals are approximated in the receiver as a random additive white Gaussian noise. This approximation is sufficient as long as interfering co-channel signals are not too strong. It is an aim of conventional cell planning to ensure this by a sufficient re-use distance.
In interference limited cellular radio systems, however, co-channel interference is typically of a deterministic nature, suggesting that it should be possible to cancel, at least partly, its effects on the desired signal.
Previously, interference cancellation methods have been developed for use in code division multiple access (CDMA) systems, which are interference limited type. It is, however, much more difficult to apply interference cancellation techniques to TDMA systems. The Finnish patent publication 944736, discloses a method for utilizing interference cancellation methods in TDMA systems.
Thus, interference cancellation methods are based on detecting some interfering signals and canceling their effect in the desired signal. The interference cancellation algorithms function better, the more accurately the interfering signals can be detected. Hence, the most advantageous situation for implementation of interference cancellation algorithms is where the receiver is receiving, along with the desired signal, a number of interfering signals which are clearly stronger than the other sources of interference, and can be canceled by the algorithms.
In frequency and cell planning of conventional cellular radio systems, however, the aim is that all the interfering signals will be as weak as possible from the point of view of the desired signal. As a result, interference cancellation methods do not function in an optimum manner in systems designed by conventional methods.